The first map in A Sapper's Story marks the day it began. As the German offensive opened, the British Expeditionary Force pushed east into Belgium, up to the line of the River Dyle. Ahead of the infantry went the Royal Engineers, clearing the roads, bridges and crossings for the BEF to advance and later to destroy. Donnie MacIver and Two Section are with them. 

The second map is the one the Sappers were made for. The advance east could not hold, and as the British Expeditionary Force began to fall back from the Dyle, the bridges it had crossed had to come down behind it. At Gastuche that task fell to Two Section, who blew the bridge over the river on the 14th of May and then worked through the night to throw two lorry piers across in its place. It was on this same crossing, over those days, that 2nd Lt Richard Annand won the first Victoria Cross of the Second World War. Donnie MacIver and Two Section were there for all of it.

As the British Expeditionary Force fell back from the Dyle, the 59th Field Company pulled out of Huldenberg and joined a vast convoy grinding west through Brussels and into the Sonian Forest. There they went to work on the forest itself. On 16 May, Two Section and their comrades prepared the trees with explosives, ready to bring them down across the tracks and hold up the Germans coming on behind. It was cold, deliberate craft: read the ground, deny it to the enemy, and buy the army time it could not spare. Donnie MacIver and Two Section are in it.

Falling back towards the River Dendre, the 59th Field Company worked the line at Ath, preparing culverts and bridges for demolition on 17 May and blowing them on the 19th once the last troops were across. Behind them, one section stayed on in the Sonian Forest to spring the trap they had laid, blowing the prepared trees in the early hours of 19 May before slipping away. By the 19th the scattered sections had rejoined the Company at Tournai. This is the rhythm of a fighting retreat: prepare, wait, destroy, move, and do it again at the next river. Donnie MacIver and Two Section are in it.

On 20 May, the 59th Field Company pulled out of Tournai in a long column and turned west for the French border. A German reconnaissance aircraft found them on the road, and the Stukas that followed struck the column and the civilian refugees crowding the same roads. Two Section broke off towards Orchies and began the slow, deliberate work of a fighting withdrawal, culverts and bridges prepared and blown behind them, denying the roads to the advancing Germans. By 22 May they had reached Pont-à-Marcq, having bought the retreat a little more time with every crossing they brought down. Donnie MacIver and Two Section are in it. 

On 22 May, Two Section turned north from Pont-à-Marcq towards the Lys, working their way up through Fretin, Gruson and Toufflers by leapfrog, one party pushing on while another held and prepared the ground behind. From Fretin they watched the Stukas work over Seclin airfield to the west. At Toufflers the road was cratered and impassable, so they repaired it and kept the column moving, then carried on north to Rekkem, helping the East Surreys back towards the Ypres-Comines line as they went. It was the plain, unglamorous craft of the Corps: keep the army moving, whatever stands in the way. Donnie MacIver and Two Section are in it.

Two Section crossed the Lys at Menin on 23 May and moved up towards Warneton, preparing demolitions and field defences along the canal line as they went. At Comines they readied the bridge for destruction alongside the 225th Field Company, then rejoined the 59th outside Warneton. This was the ground where the retreat would be fought to a standstill, and the Engineers were laying its foundations, bridge by bridge, before the blow fell. Donnie MacIver and Two Section are in it.

On the evening of 27 May, with the front giving way along the Ypres-Comines Canal, three Royal Engineers field companies, the 7th, 59th and 225th, were thrown into the fight as infantry. Engineers by trade, they held and counter-attacked alongside the infantry so the rest of the army could keep moving towards the coast. Around sixty of them were killed, wounded or missing. It is the hardest day in the book, and one of the strongest reasons it was written. Donnie MacIver and Two Section are in it.

Pulling north-west from Warneton, the 59th Field Company blew two bridges in the town on the morning of 28 May, then pressed on through Furnes towards Wulpen, joining the 7th at a woodland halt on the way. Along the perimeter they set machine-gun positions for the Middlesex Regiment, blew the barges on the canal, and made the last bridges ready for demolition. Every crossing they destroyed was one the enemy could not use to reach the beaches. Donnie MacIver and Two Section are in it.

At La Panne the men waiting on the beach had no pier to reach the boats. So on 30 and 31 May the Sappers made one. They drove lorries out into the sea at low tide, lashed them together and decked them over, building piers from the vehicles themselves so troops could walk out to the waiting craft. It is one of the most extraordinary pieces of improvisation in the whole evacuation, and it is pure Sapper: handed an impossible problem and the wrong materials, they solved it anyway. Donnie MacIver and Two Section are in it.

This is the whole picture: Operation Dynamo, 26 May to 4 June 1940, and the shrinking perimeter that made it possible. The Sappers were there from the first bridge on the Dyle to the last pier at La Panne, clearing the way in and destroying it behind, holding the line as infantry when they had to. Follow the Sapper through all eleven maps and you have followed the retreat itself, from the border to the sea. That is the ground A Sapper's Story is built on. Donnie MacIver and Two Section carried you through it.