>> reporter: ceri and rory's california home is littered with toys and medical equipment.. the rare genetic disease slowly killing the motor neurons in their daughter's spinal cord. >> you're such a brave girl. you didn't even cry when they put the iv in. >> reporter: they're now waiting to find out if the most expensive drug in the world, worth $2 million, will help save her life. >> it's a couple days after infusion, and rhys is just having a crummy day. and it just sucks. and i just feel bad that i can't help her. >> reporter: were you nervous that it might not work? >> i don't know if we were nervous that it wouldn't work. i never kind of let myself think that. >> reporter: as the couple waited to find out if zolgensma worked for rhys, ceri continued to document the highs and lows on social media. >> look at you go, rhys. >> reporter: she also relied on a facebook group for s.m.a. parents. >> my name is erin, and i'm a mother to emma. she is a 4-year-old with type 1 spinal muscular atrophy. >> my husband and i have two kids. >> our daughter hayden has spinal muscular a

i am not going to lie, cerys, i feel a bit unnatural here. i look the part...ere we go. i am on the red string... and you get the hand on the other red string, and you play them, and you've got a little echo. plays harp. like that, and back down again, then here... it's really messing with my eyes, how do you do this without going mad! that was the first bar of a famous welsh tune called pwt ar y bys, which is the one everyone learns first, so you are well on the way. you'll make a star of me yet, cerys. what a trip! wales has made me feel well and truly welcome. and that's what's really struck me at every turn during this journey — the openness of the people i've met, their history and their hope for the future. next time, in the final leg of our uk series: christa is in northern ireland, where she will take to the skies... try her hand at oyster shucking... ..and explore one of the country's magnificent natural wonders. incredible. legend has it that it came about because of a fight between two giants. it feels one where you see some sunshine today. that will