The story begins when Banban Akaza arrives on Earth to the city of Megalopolis. He is transferred to the S.P.D. (Special Police Dekaranger) Earth branch by his commander. By that time, Agent Abrella, an Alienizer arms dealer, turns his destructive attention to Earth, helping the criminals there. It's up to the Dekarangers to stop them and bring them to justice. The SPD of Earth are stationed in the Deka Base under the command of Doggy Kruger.
Dekaranger did the detective series very well. It had a lot of stand alone cases, but every now and then Abrella was involved directly or indirectly with the episode's plot. When it was time to reach the series' climax they already had a big plot going on and didn't need to come up with something out of the blue, which normally ruins a series. Deka starts a bit slow, but it soon gets interesting.
The songs were pretty good, even for Super Sentai standards.
The biggest problem in Dekaranger are the Dekarangers. Despite the fact that they are always saying that they are pros, they are completely disorganized as a team and almost never rely on strategy or tactics to beat the villains. It was too common for them to beat an enemy with willpower or stuff like that. That's a crime in a series about a special police. They didn't even have a team leader. And no, DekaRed wasn't a leader.
Dekaranger had, at the same time, awesome and terrible characters. DekaRed Akaza BanBan (codename Ban), is a hot headed and immature guy. Not Red material, specially in a series about a police force. Pink Koume Koudou (Umeko) is also immature and clumsy. The sixth arrives as an elite and arrogant policeman, but soon gets all meek and calling everybody senpai. At least he has a more professional attitude and his disguise skills are interesting. DekaGreen, Sen'ichi Enari (Sen-chan) isn't a bad character, but he isn't cool either. He's the smart guy of the team and is a bit strange. Swan-san is an alien that acts as chief scientist of the Earth branch. She's calm and acts as the mother figure.
Overall, Dekaranger is a good series. It could've been much better if they had made the Dekarangers more professional.