This set utilizes Plusle's decent offensive power and access to Encore to make him an effective Nasty Plot Baton Passer that can pose an offensive threat at the same time. Encore allows Plusle to lock defensive Pokemon, such as Registeel, into a harmless move like Stealth Rock, effectively giving Plusle a free turn to set up. As Plusle is extremely frail, Encore is often very important to obtain a Nasty Plot boost. Thunderbolt gives Plusle a reliable attack, which prevents him from being completely useless if he gets Taunted, and is fairly powerful after a Nasty Plot.
This EV spread is simple yet effective: A Timid nature and 252 Speed EVs are needed to tie with other positive base 95 Pokemon such as Houndoom. This gives Plusle a 50% chance to Baton Pass the boost in a tight situation against them, or KO the opposing Pokemon with a Nasty Plot boosted Thunderbolt. 252 Special Attack EVs boost the power of Plusle's Thunderbolt, and allow Plusle to strike the opposing team with strong attacks. While it may seem odd to boost Plusle's Special Attack when Thunderbolt is his only offensive move, the Electric-type boasts respectable mono-coverage in UU, and any opponents that resist Thunderbolt can easily be Baton Passed out of. However, some defensive EVs can be transferred from Special Attack to make it easier for Plusle to obtain a Nasty Plot boost, but so many EVs are required that Plusle becomes pitifully weak, which makes this a lesser option.
Plusle has a couple of other moves that he can use on this set. Agility is always useful to Baton Pass, but Nasty Plot is what makes Plusle unique. You may, however, forgo the use of Thunderbolt in favor of Agility if you wish. Magnet Rise can be an interesting option to Baton Pass, but like Agility, it should only go over Thunderbolt. It should be noted that replacing Thunderbolt with either Agility or Magnet Rise leaves Plusle rendered utterly useless by Taunt, so it isn't recommended. Hidden Power Ice allows Plusle to hit Ground-types he can't hit with Thunderbolt, but there are no moves that you can really replace on this set.
As this set's main goal is to pass Nasty Plot boosts, appropriate recipients are naturally important teammates. If possible, these Baton Pass recipients should also be able to deal with Ground-type Pokemon. Therefore, Moltres, Mismagius, Rotom, and Sceptile are all excellent partners, as they benefit greatly from a Nasty Plot boost, and are immune or resistant to Ground-type attacks. All of these Pokemon pack incredible powerful after a Nasty Plot boost, and are fast enough to attempt a sweep.
Because both Plusle and his Baton Pass recipients are specially-based, dedicated special sponges such as Chansey can pose problems. Even after a Nasty Plot boost, Baton Pass recipients may still face problems with the pink blob. Dugtrio is a great choice to trap and KO Chansey in one fell swoop. U-turn users, such as Scyther and Swellow, also help weaken the opposing team and scout for any Pokemon that may pose a problem to your Baton Pass recipient.