The pinnacle of Ratchet & Clank. Witty storytelling, genuinely fun and challenging time-based puzzles for Clank gameplay, an all-around excellent arsenal with new and returning weapons to utilize, great character development, a surprising twist ending, there is so much to love about this game. Only knock off a few points for the space exploration, which is really fun for 80% of the game, but definitely overstays its welcome by the end. Right up there with UYA, the best in the future franchise, and the last truly amazing Ratchet & Clank game.

A Crack in Time follows on from the first R&C Future Game Tools of Destruction's story having gameplay elements that evolves the franchise. You may want to read up on Quest for Booty to fill in a plot hole between the two games.
The first major change is the space piloting. It is done in an open world setting where you can land on mini planets to complete challenges rewarding you upgrades. This was vastly improved from ToD even making the spaceship more manageable. This is a good idea as it makes the whole R&C playable areas far more interactive.
The main 'gimmick weapons' to traverse the stages have changed, showing real innovation of the franchise. However, there was less focus on platforming and removed gliding, but added hover boots to increase mobility and speed in the larger levels.
There are slightly fewer levels but they larger giving more depth to each world. The levels using Clank have been greatly expanded upon with more engaging puzzles which was welcomed. The graphics took a small yet noticeable upgrade once again which felt like playing a low budget Pixar short film in the 2000's.
I really enjoyed the game but slightly prefer ToD. It could be the charm but also likely due to upgrade system in weapons and weapon variety. As A Crack in Time I felt the weapons, although diverse, were limited in how practical they can be. Both are amazing games and worth 20 hours of your time.