The player character arrives at Snowpoint City after finishing things off at Lake Valor. After entering the Acuity Lakefront, the player notices Barry defeated by the hands of Jupiter. After taking Uxie from Acuity Cavern, Jupiter informs the player that Team Galactic's headquarters is in Veilstone City, before leaving. After the defeat of Team Galactic, Uxie returns to Lake Acuity, where the player can battle and capture it.
Similarly to the game plot, Jupiter arrives at Lake Acuity in order to capture the Knowledge Pokémon, whereupon she is met by resistance from Platinum Berlitz, Gym Leaders Candice and Maylene. However, the three Trainers are unable to stop the Galactic Commander's advance, and she makes off with Uxie.
Lake Acuity is the only lake out of the three lakes of Sinnoh not to explicitly feature a confrontation with Team Galactic against the player character. This is because Barry split up with Dawn and Lucas, arriving at Lake Acuity. Whilst here, Barry was defeated by Galactic Commander Jupiter.
The literal meaning of Acuity is "sharpness or acuteness of wit", which is a fitting title, namely due to the fact that Uxie is the Being of Knowledge.
In an episode of the Pokémon anime, it is revealed that Acuity and the other Sinnoh lakes are in fact portals to a separate dimension, where the Lake Guardians reside.