Traffic is at a standstill after an early-morning crash on a busy dual carriageway, near Usk in Monmouthshire, A man driving a Vauxhall Astra has died after his car collided with the rear of a lorry, and Pc Richie Wyatt has been called in to investigate. This scene is littered with debris, and despite a painstaking process to work through it, there are few clues to explain this fatal loss of control.

It's now key to check out the lorry driver's version of events – and focus turns to vehicle tracking data to reveal the true picture of its movements in the lead-up to the collision. But when officers seize dashcam footage captured from the opposite side of the carriageway, it only adds to the mystery of why the driver lost his life.

Sergeant Cath Raine is on duty when a woman riding an electric scooter is seriously injured in a crash on her way to work in Newport. She's been struck by a car, and with the driver under arrest, all eyes are on Cath to work out what's happened. But when it emerges that the collision has been captured on camera, she's making no assumptions and insists that all viewpoints are considered.

When a crash triggers an automated call from a tourist's motorbike, paramedics and police are quickly alerted to their location in Raglan, Monmouthshire. As the seriously injured rider and pillion passenger are on their way to hospital, PC Richie Wyatt arrives at the scene. But the challenges posed by the tight bend of this slip road mean he's no stranger to investigating here, and the pressure's on to find new evidence relating to this latest collision.

A huge emergency response is underway on an industrial estate in Croespenmaen in Caerphilly. Two lorries have collided – and while one driver has managed to escape unhurt, the other driver is now seriously injured and trapped in his cab. As desperate efforts to free him get underway, critical care doctors arrive by helicopter, and Dean Burnett and Sergeant Rhys Dickinson are called in to search for answers.

With the amount of activity going on at this volatile scene, they're in no doubt that they could lose vital evidence and must begin a race against time to establish how the two trucks crashed. But as the evidence is unpicked, one driver's story starts to unravel.

PC Richie Wyatt is on a residential street in Risca, Caerphilly, after a powerful sports motorcycle collides with a brick wall. Two men were travelling on the bike at the time, and they have both been seriously injured. Close examination of the damage to the bike reveals the dynamics of the collision, but it's not proving straightforward to establish why the rider lost control.

In Chepstow, air ambulance doctors are treating a seriously injured woman after the car she was travelling in collided with a stationary lorry. She has been cut free from the wreckage, but as Sergeant Bob Witherall arrives at the scene, the damage quickly reveals the true extent of the impact.

The collision team investigate a late-night crash on an unlit dual carriageway. They are stunned to discover the seriously injured man was travelling on an electric scooter.

A 52-year-old cyclist has died after being knocked off her bike by a white van as she raced on a dual carriageway near Raglan in Monmouthshire. She was just nine minutes into a time trial with a group of other competitors when her ride was cut tragically short. With the van driver now under arrest and facing serious questions, Sergeant Cath Raine has been called in to investigate the final moments of the cyclist's journey. But the collision has happened on a concrete road, with barely a trace of evidence on its surface – and she knows that even a single mark could hold the key to justice. So when she finally discovers a clue at the edge of the carriageway, it raises more questions than answers.

A road worker is hit by a modified car during a police pursuit. However, when the driver fails to stop, the race is on to track the car down and bring the offender to justice.