On their 20th birthday, identical twin astronauts volunteer for an experiment. Terra will remain on Earth while Stella boards a spaceship. Stella’s ship will travel at 86.6% the speed of light to visit a star that is 10 light-years away, then return to Earth at the same speed. As they prepare to part ways, the twins wonder what will happen when they’re reunited. Since a light year is exactly the distance light can travel in a year, Stella’s journey should take 23 years. But from having studied special relativity, the twins know it’s not that simple.