Keiichiro/Patren #1 of the International Special Police's tactical unit and his team infiltrate a casino being illegally run by the Gangler. There they come in contact with the rambunctious Kaitou Sentai Lupinranger. Trading blows with Gangler member Galat-Nago, the Lupinranger steal Lupin's treasure, which is part of the "Lupin Collection" from the Gangler, defeat the Gangler, and gain some acclaim for doing so.
The Gangler are a crime syndicate from another dimention while the Lupin Collection is a collection of treasures stolen by the great thief, Arsene Lupin during his lifetime. The collection is said to contain mysterious artifacts with strange powers. The Lupinranger are tasked to reclaim the treasure and end up trading blows with Keiichiro's team, the Keisatsu Sentai Patranger. Keiichiro is scolded for having a problem with "thieves stealing from thieves", but maintains his stance that a "thief is a thief" and mourns his team's lack of power to stop them.
With Galat having escaped from the teams, the Lupinranger and Patranger clash with each other.
Now for some more stuff about the series, the Lupinranger has a day job working in a bistro called "Gelée". As for the Good Striker, it is part of the Lupin Collection and is alive with his own will. Similar to Engine Sentai Go-Onger's Engine mecha, this mecha is sentient and he will be voiced by Yuuji Mitsuya (Dragon Ball Z). As for the Gangler, they've been active fro 500 years.
[Source: Dukemon of Rangerboard (link 2), @Leg10nOfBoom]