Splatoon 3 Story Mode Walkthrough — Return of the Mammalians Complete Guide (All Levels, Bosses & Secrets)

Splatoon 3's single-player campaign, Return of the Mammalians, takes place in Alterna — a snowy wasteland filled with human ruins, fuzzy ooze, and kettle levels. You play as Agent 3 (new recruit) alongside Smallfry buddy. The structure: 7 sites (islands) in Alterna, each with kettle levels, a boss kettle, and hidden collectibles. This walkthrough covers every site in order with boss strategies and the secret final level.

How the Campaign Works

The hub is Alterna — you walk around snowy terrain finding kettles (levels). Each site has:

• 7-12 kettle levels (main challenges)

• 1 boss kettle at the end

• Hidden items: Sunken Scrolls, Sardinium, Gold Records, Decorations

• Power Eggs scattered everywhere (use Smallfry to grab them)

Clearing a kettle removes fuzzy ooze nearby, unlocking more areas. Kettles give you a specific weapon or weapon set — you can't bring your own gear. Power Eggs unlock upgrades at the upgrade station. Sardinium is the rare upgrade currency.

Site 1: Future Utopia Island

Your tutorial site. Easy levels teaching basic mechanics.

Key Levels:

• Kettle 1: Teaches ink swimming, shooting, and basic platforming with the Splattershot

• Kettle 3: First Splatana level — use charged slashes to break crates and kill Octarians

• Kettle 5: Introduces the Tri-Stringer — shoot targets across gaps, learn the arc

Boss: DJ Octavio (Round 1)

First fight against the series villain. He pilots a giant mech with punching fists. Dodge the fist slam (it leaves ink circles on the ground), then shoot the glowing weak point on the fist when it's stuck in the ground. Repeat three times. Very easy — it's a tutorial boss.

Collectibles to grab:

• Sunken Scroll 1: On a high ledge to the left of the boss kettle entrance

• Sardinium x2: One behind a fuzzy ooze block (need Power Eggs to clear), one visible on a pillar near Kettle 4

Site 2: Cozy & Safe Factory

Industrial theme. Introduces moving platforms and conveyor belts.

Key Levels:

• The Splatling level here teaches you to charge behind cover, then step out and fire

• Zipcaster special introduction — grapple across gaps using ink trails

Boss: Octostomp

Classic boss returns. It charges at you in a straight line, then flips over revealing its weak point (the tentacle on its back). Dodge the charge, shoot the tentacle when it's stuck on its back. Phase 2 adds ink trails behind the charge — stay on raised platforms. Phase 3 adds two mini Octostomps that charge simultaneously.

Site 3: Cryogenic Hopetown

Frozen city ruins. Introduces advanced platforming and the Trizooka special.

Key Levels:

• Rail grinding level — shoot targets while riding rails, jump between rail gaps

• Zipcaster platforming combo — grapple, ink a wall, swim up, grapple again

Boss: Octowhirl

A spinning top enemy that bounces around the arena. When it hits a wall, it stops and exposes the weak point. Use bombs to stun it, then unload. Phase 2 adds two Octowhirls. Phase 3 adds ink trails behind their movement paths.

Site 4: Landfill Dreamland

Garbage dump aesthetic. Introduces Splashdown special and more complex combat arenas.

Key Levels:

• Inkbrush stealth section — sneak past searchlights, kill enemies silently

• Limited ink level — you can't refill, must manage every shot

Boss: Octomaw

A giant robotic shark that swims through ink. It surfaces to attack, then submerges. When it surfaces for a bite attack, shoot the weak point in its mouth. Phase 2 gives it armor plates — shoot the plates off first, then the weak point.

Site 5: Eco-Forest Treehills

Lush forest with verticality. The hardest platforming levels are here.

Key Levels:

• Zipcaster + rail combo — chain three zips, three rails without touching ground

• Charger-only level — moving platforms with distant targets, 20 shots to clear

• The infamous Soaker Block level — shoot blocks to create platforms, single mistake means restart

Collectibles:

• Sunken Scroll 12: Hidden behind a waterfall near the far-left cliff edge

• Gold Record: Complete every kettle with every available weapon to unlock

Site 6: Happiness Research Lab

The final normal site. Futuristic lab aesthetic. Prepares you for the final boss.

Key Levels:

• All-weapon gauntlet — four rooms, switch weapons between each, test of everything

• Target range speedrun — clear 30 targets in under 2 minutes for maximum Power Eggs

Boss: Mr. Grizz

The final boss and the best boss fight in Splatoon history. This is a multi-phase cinematic battle in space:

Phase 1: Mr. Grizz pilots the rocket. You ride a moving platform. Shoot the rocket's exhaust vents (orange glowing spots). Dodge rockets fired at you by moving to the edge of your platform.

Phase 2: The rocket transforms into a giant mech. Shoot the glowing spots on its arms and chest. Mr. Grizz fires ink lasers and missiles — the lasers sweep horizontally (jump over them), missiles have ground indicators.

Phase 3 (Final): Fuzzy Ooze covers the mech. Use Smallfry to eat the ooze (throw Smallfry with R), exposing the weak points underneath. While Smallfry eats, you shoot the exposed spots. This phase is about alternating between Smallfry throws and shooting — the timing is tight.

After beating Mr. Grizz, you get the credits and return to Alterna. You've beaten the main campaign.

The Secret Kettle: After Alterna

After clearing every site, a new kettle appears at Site 1. This is After Alterna — the hardest level in Splatoon 3. It has four sections in one level with NO checkpoints between sections. If you die at any point, you restart from section 1.

Section 1 (Target Range): Destroy 30 balloons in 30 seconds with the Splattershot. Precision matters — miss more than 3 shots and you're too slow.

Section 2 (Soaker Blocks): A long platforming section where you shoot blocks to create temporary platforms. The blocks shrink fast. One misstep = restart from section 1. Move fast and don't stop.

Section 3 (Octarian Gauntlet): Waves of elite Octarians including Octoling snipers and shielded enemies. Use Splashdown and Trizooka specials freely — there are refill stations between waves.

Section 4 (Final Run): A rail-grinding escape sequence with enemies on all sides. Stay on the rail, shoot targets as they appear, don't fall.

Beating After Alterna unlocks the Teddy Band headgear (cosmetic flex piece) and a title. You'll need 2+ hours and intense patience for this one.

All Collectibles Overview

• Sunken Scrolls: 24 total, one hidden in each site plus Alterna overworld

• Sardinium: Used at upgrade station for Hero Shot damage, ink tank capacity, armor

• Gold Records: Unlocked by completing every kettle with every weapon option

• Decorations: Locker decorations found throughout Alterna

• Power Eggs: Thousands scattered across Alterna — use Smallfry to grab eggs on high ledges

Upgrade Priority

At the upgrade station, buy upgrades in this order:

1. Ink Tank Level 1-3 (more shots before reloading)

2. Hero Shot Damage 1-2 (faster kills on Octarians)

3. Armor 1 (survive an extra hit)

4. The rest as you collect Sardinium

Campaign Length

• Main story (all 6 sites): 6-8 hours

• 100% all kettles all weapons: 12-15 hours

• After Alterna secret kettle: Add 2+ hours

• All Sunken Scrolls: Add 1-2 hours of exploration

Hero Weapon Replicas

Completing all kettles with all available weapons unlocks Hero Replicas — cosmetic reskins of the multiplayer weapons with the Hero branding. They look cool but have no gameplay advantage over standard weapons.

Splatoon 3 Expansion Pass — Side Order

The Expansion Pass adds Side Order, a roguelite mode where you climb the Spire of Order as Agent 8. Each floor is a randomized challenge. You collect Color Chips to build your character's abilities per run. This is separate from the main campaign and adds 10+ hours of content. The final boss is a multi-phase fight against Order. Well worth it if you enjoy the main campaign.