They're usually out saving the world but a new very important mission has been issued to The Avengers.

Iron Man star Robert Downey Jr has released a call to arms to his fellow superheroes to help a young boy from Scotland save the lives of his friends.

Eight-year-old Aaron Hunter, from West Dunbartonshire, is fighting a devastating rare disease called ROHHAD which has already caused the loss of several of his young friends diagnosed with the same syndrome.

An under-researched condition, for which there is currently no known cure, Aaron was so determined not to lose any more friends that he wrote a letter to Iron Man asking him to help save them.

Avengers fans were quick to pick up the letter and shared it widely until the actor himself read it.

Robert Downey Jr then called Aaron himself, in character as Tony Stark, and the pair have since been spending time together figuring out how to help Aaron and his friends.

The star has already begun his support by offering fans the chance to attend the world premiere of Avengers: Infinity War with him in California on April 23, 2018.

The entry fee to the competition will be donated to Aaron's ROHHAD charity, set up to support children suffering from the syndrome and to help fund research into a cure.

Robert Downey Jr has now also taken up Aaron's own challenge, a muddy puddle jump to help raise funds for the charity.

The star donned a Scottish 'see-you-jimmy' tartan hat and a white jumpsuit before dancing in a muddy puddle with some truly heroic moves.

He then shared it to Facebook and tagged all his Avengers mates and fellow Hollywood actors to jump in a puddle too for Aaron and his friends.

Chris Evans, Scarlett Johansson, Chris Hemsworth and Mark Ruffalo have all been asked, as have Samuel L. Jackson, Paul Bettany, Elizabeth Olsen, Anthony Mackie as well as Benedict Cumberbatch, Evangeline Lilly, Martin Freeman, Gwyneth Paltrow, Idris Elba and Vin Diesel.

Aaron's mother Lisa said he is "delighted" and his "already big heart is full of hope".

"This all began because of Aaron's determination to help save all of his friends around the world and his unwavering belief that Iron Man would help him to do that," she said.

"Aaron's selfless love and compassion was our inspiration. He kept us going and we knew we had to do all we could to reach the real Iron Man for him.

"The hope this has given all the families is immeasurable."