Ron Pember

A group of children try to help a tiny spherical alien get back to its mothership, while the army and a devious petty crook pursue the creature for its wonderful powers. (imdb)

During World War II, a Belgian resistance movement called Lifeline, based in Brussels, organises the return of Allied airmen who have been shot down by the Luftwaffe to the United Kingdom.

A scientist named Griffin invents a way to change his body's refractive index and thus becomes invisible. He uses the opportunity to carry out random acts of violence.