The Megalopolis Parade
Ratchet and Clank Rift Apart starts after a short cutscene showing Rivet, with Ratchet and Clank taking part in a parade. This is essentially the intro/tutorial mission and is all fairly straightforward. Grab your wrench, smash the crates, and head from one parade float to the next.
Before long though, Dr. Nefarious shows up and all hell breaks loose. This section is all entirely linear, so once you've grabbed the Burst Pistol and killed the Goons-4-Less, take part in the rail grind tutorial. From here, continue forward with the shootin' and the blastin' until you have to take down the Goon Dropship. If you're struggling here, make sure to keep your distance – the dropship is a much bigger target than you so you can shoot it from afar – and make a mental note of where the ammo and health crates are.
Surprise surprise, Dr. Nefarious steals the Dimensionator and all hell breaks loose. Follow along with the grind rails until another cutscene sees you land next to the first Mrs. Zurkon, the weapons vendor.
Megalopolis Train Station
The next section of Megalopolis takes place in and around the train station and this is where it starts to open up a little. Don't worry about collectibles here though, the only important things to find are standard crates and some raritanium. Speak to Mrs. Zurkon to buy the Shatterbomb, a glove that throws insta-detonation grenades.
Defeat the Goons-4-Less in the area next to Mrs. Zurkon, then progress up the stairs on your right-hand side. The train will zoom off and you can cross into a circular room with a tree. Ratchet will shoot at Dr. Nefarious in a cutscene, hitting the Dimensionator and opening rifts, but now you can use rift tethers by pressing L1. Jump to the first one then before you go left, look right; there's a rift tether to jump to by your first few crates of raritanium.
Head back over and jump to the rift tether on the left, then clear all the Goons-4-Less from this area. Plenty more crates and bolts to obtain here if you look around, then go inside the building and deal with the Sandsharks that come through a portal. Keep going and head through the next door on your left to find a stack of crates, then continue the way you were for another cutscene to happen.
In this next area, smash the stack of crates against the far wall, then head up the rift tether on the moving crane. When this reaches the top and you jump to the next one, turn to the left and glide over to the platform, where you can then rift tether up to a small cranny with more raritanium inside. Head back to the rift tether on the crane and jump to the next one through the descending crates to continue.
Outside the next building you have more Goons-4-Less to kill, but before you jump on the taxi, make sure you head through the corridor to your left to find a bunch more crates to smash. Then hang on to the taxi with L1 and you'll be dropped off by another Mrs. Zurkon, where you can buy The Enforcer. This is essentially a shotgun with some serious upgrade potential.
Clear the Goons-4-Less from the next area – there's a lot of them in this bit – and you'll have the first fight against Dr. Nefarious. Your Shatterbomb will come in handy here, along with the Blast PIstol. The fight isn't too difficult but make sure you're constantly strafing to avoid the missiles, jumping over the laser beam, and when he moves away, smaller enemies are going to spawn through portals so be ready for them. Once Dr. Nefarious is "defeated", this is the end of Megalopolis.