http://www.youtube.com/watch?v=CcFj3CfBSbU <= last part of first episode, pretty much explains the story.
http://en.wikipedia.org/wiki/UFO_Baby 

This is a lighthearted anime about a 9th grade girl named Miyu whose parents work for NASA. Her parents have to leave for a special mission/project so she has to stay at the Saiyonji shrine. The shrine owner's son, Kanata is her age. Kanata's father gets a call and suddenly has to leave for an important trip as well, leaving the two living together at the shrine. The two don't have a good first impression of each other, and drive each other nuts the first day. All of a sudden a crash is heard in the shrine and they go to investigate. They find a spaceship with a flying alien baby inside. The baby, named Lou, thinks that Miyu and Kanata are his parents. Along with Lou, is his "sitter-pet" (an animal-nanny that takes care of children), Wannya ("wan" is the Japanese equivalent to "woof" and "nya" is the japanese equivalent to "meow"). They come from the planet, Otto, and somehow got sucked into some kind of time/space hole that transported them to the Earth. Unfortunately for them, their space ship is severly damaged and they're stuck on Earth until they can fix it. The story centers around the two teenagers taking care of their new friends, trying to conceal their identity, despite all these crazy situations that threaten to blow Wannya and Lou's cover--like Miyu's parents going out on a search for Aliens to dissect for scientific research, and other people who become suspicious as to the true identity. It's hard not to get into sticky situations with interesting characters like Christine, this girl with a crazy obsessive crush on Kanata, Mikan the highstrung manga artist always looking for inspiration, and other interesting people. As time goes by, Miyu and Kanata grow attached to Lou feeling more and more like they have a responsibility to him, like they are his parents. Will everyone find out that Lou and Wannya are really aliens? Will they ever get back to planet Otto? And what's going to happen when Lou figures out that Miyu and Kanata aren't his parents?

I watched this in the original Japanese way back in middle school, but have recently discovered that there is an English dub. I like this anime because it's so lighthearted. Yes, it's girly, yes, a lot of guys will be thinking, "Oh God, not another cheesy girly show." But hear me out! I knew a couple of guys who secretly get hooked to this cute show. Why? It's hard to resist babies and the cute things that they do, and it's definitely hard not to fall in love with the characters, the main characters and their quirky, interesting friends.

I think it needs to be put on to cater to the female audience (we love and grew up with guy shows, too, don't get me wrong, but we need to satisfy our dose of cuteness (without being too hello kitty or pucca girly)). It's a break from the typical magical girl superhero type of shoujo. (I am a die hard Sailor Moon fan, and I loved Cardcaptors, too, don't get me wrong there, either). There's funny situations, times were you are in light suspense (oh no! they're going to find out about Lou and Wannya!), cute young romance, love triangles, and other things. It's along the lines of a romantic-comedy kinda rated PG/G family-friendly feeling kinda show, if that makes sense.

I really think that this is a show that will grow on both guys and girls, both kids, the young adults who refuse to grow up (that's why we still love Toonami), and those who are young at heart. Once you get into it, you can't wait to see what happens next! (I challenge you to watch 3 episodes and tell me that you weren't tempted to keep moving on to the next episode.)