Transitions with borders and can be done many different ways. The question implies you're quite new to all of this and asking about a plugin implies you're hoping for a shortcut. I suppose there are some, try hitting CoreMelt's page and FxFactory. Sophisticated plugins are not free. You can do most of those in FCPX but you must learn how the animation systems function and are programmed. Not easily explained in these little posties. Try searching youtube for "motion graphics in FCPX" and "animating position parameters". The FCPX help system is great for explaining the use of animation keyframes.
The coolest effect I saw in your sample was around 05:15 where the group portrait appeared to be partially animated. Can't really tell if the moving couple on the right were inserted or just refused to stand still.
Making this is actually quite simple but you'd want Apple's Motion 5 app for that:
1) open a new Transition project ,
2) extend both Transition A (video you're transitioning FROM) and Transition B (video you're transitioning TO) layers to match the project duration,
3) make sure that the Transition B layer is above Transition A,
4) animate the Transition B layer's "Position X" parameter (select layer -> Inspector -> Properties -> Position) so that it slides in from outside the screen,
5) enable the Motion Blur feature via Render menu.
After publishing (saving) the template it will show up in Final Cut as a new transition that can be simply dragged between your clips!
You can also fake this in FCPX alone by animating the second video via the app's built in keyframing tools and applying a horizontal Directional Blur effect with an animated amount on top.