| Talented and successful surgeon Daniel Milton has his world shattered when his wife Beth develops a fatal degenerative disease. After conventional treatment options run out, Daniel partners with the lonely yet resourceful Lee to start a literal Underground clinic in the vast network of tunnels beneath Temple Tube Station in London. |
| Name | Character | Episodes |
|---|---|---|
| Mark Strong | Daniel Milton | 15 |
| Carice van Houten | Anna | 15 |
| Daniel Mays | Lee | 15 |
| Chloe Pirrie | Karen Hall | 8 |
| Marion Bailey | Ingrid | 8 |
| Lily Newmark | Eve | 8 |
| Siena Kelly | Michelle Wilson | 6 |
| Corin Silva | Kieran | 1 |
| Jack Colgrave Hirst | Greg | 1 |